This invention relates to the design and structure of electronic filters, and more particularly to an electronic filter implemented in an integrated circuit having a substantially reduced die area as compared with prior art filters.
Filters are well known in the prior art. Such filters serve as means for rejecting signals above a selected frequency (low pass filter), rejecting signals below a selected frequency (high pass filter), rejecting signals of a selected frequency (notch filter), or rejecting frequencies other than those within a predefined frequency range (band pass filter). Oftentimes, it is desirable to sample an analog signal; for example, for use in a sampled data system. Such sampled data systems are widespread and provide the advantages of allowing a digital circuit to perform analog functions. One of the problems associated with such sampled systems is known as "aliasing". Aliasing is the undesired effect that, in addition to passing selected frequencies within its desired pass band, a sampled data system will pass signals having a frequency near the sampling frequency.
FIG. 1 illustrates this aspect of aliasing. FIG. 1 shows a frequency domain graph of a sampled data system in which it is desired to pass audio signals within the frequency range of 0-3 khz. Thus, the filter characteristics are such as are shown on the left-hand side of FIG. 1. However, when the sampled data rate is 128 khz, the same transfer function is provided on both sides of 128 khz. Thus, signals within the range of 128.+-.3 khz are passed by the sampled data system. Further, these signals appear at the base band by virtue to being "mixed" with the 128 khz clock frequency. It is therefore desirable to eliminate the presence of such high frequency signals prior to being fed to the sampled data system. In this manner, although the sampled data system will have little or no attenuation of 125-131 khz signals, such signals will not be present in the input signal provided to the sampled data system, and thus are removed from the filter output. In order to accomplish this, oftentimes a low pass prefilter, or "antialiasing filter", is utilized to filter out such high frequency components of the input signal prior to applying the filtered input signal to the sampled data system. The graph of the transfer function of a typical antialiasing filter is shown superimposed on the graph of FIG. 1.
Such antialiasing filters are well known in the prior art and typically comprise a simple low pass filter. In many instances, a fair amount of attenuation is desired, and thus a two-pole filter is utilized. One such two-pole filter is shown in the schematic diagram of FIG. 2. Antialiasing filter 20 includes operational amplifier 26 having its inverting input terminal connected through resistor 27 to ground and also through resistor 28 to output terminal 29. The noninverting input terminal of operational amplifier 26 is connected through capacitor 25 to ground and through resistor 23 and resistor 22 to input terminal 21. The node between resistors 22 and 23 is connected through capacitor 24 to the output terminal of operational amplifier 26. The transfer function for the two-pole low pass filter 20 is as follows: ##EQU1## where EQU b.sub.0 =1/(R.sub.22 R.sub.23 C.sub.24 C.sub.25) EQU b.sub.1 =[(1-G.sub.1)/(R.sub.23 C.sub.25)]+1/(R.sub.22 C.sub.24)+1/(R.sub.23 C.sub.24) EQU G.sub.1 =1+R.sub.28 /R.sub.27
where
H(s)=The transfer function of antialiasing filter 20; PA1 R.sub.22 =the resistance of resistor 22; PA1 R.sub.23 =the resistance of resistor 23; PA1 R.sub.27 =the resistance of resistor 27; PA1 R.sub.28 =the resistance of resistor 28; PA1 C.sub.24 =the capacitance of capacitor 24; PA1 C.sub.25 =the capacitance of capacitor 25; and PA1 G.sub.1 =the closed loop gain of operational amplifier 26.
In practice, the values of resistors 22, 23, 27 and 28 and capacitors 24 and 25 are selected according to well-known principles, such as are stated in "Rapid Practical Design of Active Filters" by D. E. Johnson and J. L. Hilburn, John Wiley & Son, (particularly pages 12, 13 and 27-29), which is hereby incorporated by reference.
Oftentimes, it is desired to fabricate an entire sampled data system as a single integrated circuit device. In this case, it is possible to provide an antialiasing filter as shown in FIG. 2 external to the integrated circuit, although this is undesirable. Therefore, it is desirable to fabricate the sampled data system, together with one or more antialiasing filters such as the antialiasing filter 20 of FIG. 2, as part of the same integrated circuit. While this has been done, the fabrication of such antialiasing filter 20 in FIG. 2 on an integrated circuit requires a considerable amount of die area, particularly when it is recognized that oftentimes a single sampled data integrated circuit requires four or more such antialiasing filters to be fabricated on the single integrated circuit chip. Prior art techniques for fabricating such antialiasing filters utilize portions of the polycrystalline silicon interconnect layer as resistors 22, 23, 27 and 28. While the use of polycrystalline silicon resistors is well known, and easily implemented in order to fabricate the antialiasing filter 20 of FIG. 2, the use of polycrystalline silicon resistors requires a substantial amount of die area. Polycrystalline silicon resistors typically have a resistance of approximately 20 ohms per square. Typically resistor values needed are within the range of approximately 50K .OMEGA. to 400K .OMEGA.. Using polycrystalline silicon requires a large number of squares (for example, 1 square of polycrystalline silicon may be designed to be 8 microns square); further, resistors are spaced at least 5 microns apart and thus require a large area. Table 1 shows an approximate area calculation for a voice band filter having a closed loop gain of unity. Thus, the prior art antialiasing filter 20 of FIG. 2, fabricated on an integrated circuit device utilizing prior art techniques including the use polycrystalline silicon resistors, and allowing approximately 10% additional area for proper spacing and interconnection, requires approximately 1730 sq. mils per antialiasing filter. This is a substantial amount of integrated circuit device area, and substantially limits the number of integrated circuits which may be formed on a single semiconductor wafer. As is well known, the smaller the die area required for an integrated circuit, the more integrated circuits may be fit onto a single semiconductor wafer, and the greater number of finished integrated circuits possible for a given amount of semiconductor processing. This, of course, decreases the cost of each finished integrated circuit device.